- Prefer using constants to new strings

- Prefer using String.isEmpty() to String comparison with ""
- Prefer using DeviceProxyFactory.get(device_name) to new DeviceProxy(device_name)- Respect SOLEIL convention
- Prefer using "for each" to "for int in list size"
- Avoid return inside if
- Don't select context if loading failed
- TANGOARCH-629: Ensure "register" button is deactivated when context is loaded + register a new context if attribute list changed